Influence of coastal effects, bottom topography, stratification, wind period, and mixing upon the propagation of internal waves and the profile of wind‐induced currents

Abstract

[1] A three-dimensional model in cross-sectional form is used to examine the influence of wind period, stratification and bottom topography upon internal wave generation and wind-induced current profiles in the coastal ocean. Calculations with fixed vertical diffusion coefficients and with a turbulence energy model are performed. These show that besides the wind period influencing the depth of penetration of the wind's momentum, as in a point model, it determines the offshore extent of internal waves generated at the coast. These waves together with elevation gradients modify the directly wind-driven current profile found in a point model. For forcing at the subinertial frequency the internal wave field is trapped in the coastal region. Offshore the current can be derived from a point model incorporating wind forcing and an elevation gradient related to it. For wind forcing at the superinertial frequency the offshore extent of internal wave propagation increases with frequency. The local topography influences the offshore propagation of internal waves, leading at superinertial frequencies to regions of enhanced surface current magnitude in a similar manner to that found with internal tides. Calculations with the turbulence energy model show increased surface diffusion in these regions, which allows wind energy at both superinertial and subinertial frequencies to penetrate to depth in offshore areas. This process does not involve subinertial internal wave propagation. Conclusions are drawn as to the conditions under which a single point model can reproduce wind induced current profiles at an offshore location.INDEX TERMS: 4219 Oceanography: General: Continental shelf processes; 4512 Oceanography: Physical: Currents; 4544 Oceanography: Physical: Internal and inertial waves; KEYWORDS: internal waves, bottom topography, coastal processes, wind period, mixing, turbulence energy Citation: Xing, J., and A. M. Davies, Influence of coastal effects, bottom topography, stratification, wind period, and mixing upon the propagation of internal waves and the profile of wind-induced currents,

A three-dimensional model study of near-inertial motion: generation and influence of an along-shelf flow

“…Similarly, displacements of the density surfaces gives propagating near-inertial internal waves with a speed of order (g¢h) 1/2 with g¢ the reduced gravity (Millot and Crepon 1981). As shown by and Xing and Davies (2003), the combination of the inertial oscillations in the surface well-mixed layer, and the free surface pressure gradient associated with the surface gravity wave is responsible for producing the observed 180°phase shift in inertial oscillations across the thermocline. In addition to these inertial oscillations, nearinertial internal waves at a frequency of order 1.05f (where f is the inertial frequency) propagate away from the coastal generation region.…”

“…On the shelf the elevation pressure gradient associated with the surface gravity wave that propagates from the coast drives a significant current, giving rise to inertial oscillations at depth which are phase-lagged by 180°from the surface. For a given sea-surface elevation gradient, in the absence of friction Xing and Davies 2003), showed that the magnitude of these oscillations below the thermocline was reduced as water depth increased, due to the fact that the elevation pressure gradient acted over the whole water column. In shallow-water and nearcoastal regions, both frictional effects and coastal inhibition reduce the magnitude of the oscillations below the thermocline, giving rise to the near-bed maxima in hðu À huiÞ 2 i, shown in Fig.…”

“…3a), it is clear that there is a surface and near-bottom intensification at about 45 km from the coast that was not present previously. As shown by Xing and Davies (2003), features such as this are associated with internal waves which propagate away from the coastal region. Since in this calculation the density field can vary (i.e.…”

“…Calculations using cross-sectional models with idealized topography, together with point calculations Xing and Davies 2003) suggest that the intensity of the wind-induced currents and associated internal waves will depend upon topography and vertical stratification which will influence the current profile.…”

“…In reality, these are more likely to be the generation mechanism, as in many regions the water is well mixed at the coastal boundary. In a detailed investigation of the influence of wind period forcing, stratification and bottom slope, Xing and Davies (2003), using a cross-section model with idealized topography, showed that for wind frequencies above (superinertial) and at the inertial frequency, the internal waves propagated at a significant distance from the coast, with regions of intensification depending upon topography and stratification, in a manner similar to that found for the internal semidiurnal tide (Xing and Davies 1998). At frequencies below the inertial period, the internal wave was trapped in the coastal region (Mooers 1975;Kunze 1985;Dale et al 2001).…”